The conventional electromagnetic motor has been widely used for a large variety of applications since its development some 120 years ago. The manner of operation of such electromagnetic motors however prevents its use in the field of micro electromechanical systems. This is because such motors operate by way of an interaction between permanent magnets and a magnetic field induced by a coil. As the size of the motor is reduced, the magnetic force reduces proportional to the length scale of the motor to the power of four. Furthermore, the rotation speed increases in proportion to the motor diameter to the power of two with motors of 1 mm in diameter typically having rotation speed in excess of 25,000 rpm. Therefore it is practically not possible to use electromagnetic motors for designs having a volume less than 1 cm3.
By comparison, ultrasonic motors have the benefit of being able to maintain relatively large output torques at such small dimensions. In order for a piezoelectric transducer to function as a motor, it is preferable to induce both a torsional as well as an axial mode within an elongate transducer element. Various piezoelectric motor designs have been developed to achieve such actuation of an elongate transducer element. We refer for example to the paper titled “Improvement of the Longitudinal Vibration System for the Hybrid Transducer Ultrasonic Motor” by Jun Satonobu et al (IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 47, No. 1, January 2000), there is described an ultrasonic motor which uses a series of piezoelectric annular discs interposed between annular discs of frictional material, and together surrounding an elongate drive shaft. This arrangement allows for both axial as well as torsional vibration to be induced in the drive shaft. This ultrasonic motor is however relatively complex in design and expensive to make and cannot be used at sizes less than 10 mm in length.
It has been found that it is possible to induce both an axial and torsional vibration in an elongate transducer if an asymmetric inhomogeneity is introduced along that transducer. For example, the transducer may be produced by twisting a flat plate or square or bars of other non-circular cross-sections to produce a relatively uniform spiral arrangement along the length of the transducer. A problem with the use of such twisted transducers is that it is difficult to adequately control the twisting of the transducer during fabrication to provide the necessary twisted configuration. Therefore, the practical difficulties in producing such a configuration limits its application, particularly in micro electromechanical systems.